Solar cell installations are a model of environmental compatibility: in service they generate neither emissions, waste heat nor noise. The technical potential of solar cell installations for the generation of power is very considerable throughout the world. They can be applied practically anywhere and for any purpose, such as the decentralized supply of energy to remote settlements, centralized power generation in solar-powered vehicles.
The extent to which the potential of solar cells can be realized is largely dependent on whether the development of solar cells leads to cheaper manufacturing processes--if this proves to be the case, it is conceivable that a significant proportion of power generation may be met with solar cells, even in the industrialized nations with their low amounts of sunshine.
Ways and means of lowering the manufacturing costs of solar cells have accordingly been sought.
Solar cells are based on the so-called photo-voltaic effect. Sunlight is capable of liberating charge carriers which can be separated at the interface between two different semiconductors, so that an electrical voltage is produced. Its detailed mechanism is as follows.
Where an electron conducting or n-type semiconductor (e.g. silicon doped with trace amount of phosphorus) is situated adjacent to a hole conducting or p-type semiconductor (e.g. silicon doped with trace amounts of boron), an internal electrical field is produced in the boundary zone, i.e. the p-n transition. Negatively charged electrons migrate from the n-semiconductor to the p-semiconductor, and holes (positively charged sites not occupied by electrons) from the p-semiconductor to the n-semiconductor, in order to compensate for the charge gradient at the interface. If light, e.g. solar radiation, is now absorbed by the semiconductor, it causes pairs of electrons and holes to be released on both sides of the interface. Under the influence of the internal field, the electrons collect in the n-type semiconductor, and the holes in the p-type semiconductor, causing a photoelectric voltage equal and opposite to the internal field to occur between the two sides. Contacts can be used to tap this photoelectric voltage and supply it to an external circuit. The current intensity increases with the intensity of illumination and the size of the illuminated area. The voltage is dependent on the semiconductor materials. Commercial crystalline silicon solar cells measure up to 10 cm.times.10 cm and produce a voltage of approximately 0.5 volt and a peak power output of 1 watt. Higher voltages and power outputs are required, however, in solar cell installations. According to the prior art, manufacturers connect several solar cells one after the other in series and encapsulate them under glass in a weatherproof fashion to produce a solar module, in the form of a replaceable unit. A solar cell generator can be made by connecting together one or more modules in series or in parallel to provide the desired voltage or power output. According to the crystalline form of the semiconductor, it is possible to distinguish between three basic types of solar cells, with silicon being the predominant semiconductor material.
There are what are known as monocrystalline solar cells. Their manufacture involves drawing a p-type silicon monocrystal ingot from a molten silicon mass and slicing it into wafers of about 0.4 mm in thickness. The front face directed towards the solar radiation is doped by the diffusion of, for example, an n-type dopant, e.g. phosphorus, with a thickness of three to four microns to produce the p-n transition that is critical for the principle to function. Metal contacts for collecting the current are vacuum-metallized on both faces. In the case of the front face, this is in the form of a lattice covering not more than 10 per cent of the surface area so as to permit the transmission of the greatest possible amount of solar radiation. The largest crystals or wafers have a diameter of approx. 15 cm.
The individual solar cells are connected together in series to form a module by connecting the positive metal contact of each solar cell to the negative metal contact of a neighboring solar cell, or vice versa, by means of contact connectors. This series connection results in spatial gaps between the solar cells of a module, known as inactive zones, where the solar energy is unutilized.
Also know are polycrystalline solar cells, the manufacture of which involves casting a block of silicon consisting of a large number of small crystals, which block is then sliced and processed in the same way as the monocrystal. Its efficiency is lower than that of monocrystalline solar cells.
Finally, thin-layer cells are also known. In this case a one micron thick p-i-n sandwich (p-type conductor, undoped intrinsic semiconductor, and n-type conductor), is applied to a glass, film, or metal strip by a variety of processes, e.g. vacuum-metallization or vapor deposition. This p-i-n sandwich is neither a monocrystal or polycrystal, but remains amorphous (without structure). The presence of the i-type conductor is necessary, as excessive recombination would otherwise take place in the amorphous p-type and n-type conductors, leading to reduced efficiency. The collection of the current takes place on the front face via a transparent, conductive layer of tin oxide, and on the rear face via an aluminum contact layer.
Cells of this kind basically can be made to any desired size. The layers are applied without discontinuities. However, in order to obtain the required voltage, the layers are subsequently perforated by an expensive process involving the use of laser cutters. Both the front and the rear contact layers are perforated at regular intervals by straight grooves, and are divided up in this way into a large number of cells. The grooves in the upper contact layer are slightly offset in relation to the grooves in the lower contact layer, so that the peripheral zone of the front contact layer of a cell and the peripheral zone of the rear contact layer of the adjacent cell overlap. The two overlapping zones are connected to one another by means of a link of silver or some other conductive material, which runs perpendicular to the contact layers and parallel to the two adjacent grooves. In this way, neighboring cells are also connected in series.
The resulting modules suffer from the disadvantage that they are unable to utilize solar energy, i.e. they are inactive, in the zones of the silver links. Their manufacture is also very time-consuming and is associated with correspondingly high costs. The laser cutting machines required for perforating the layers are also very expensive. The laser cutting process also creates a lot of unnecessary and environmentally undesirable waste. The reinstatement of the links between the cells is also a costly and expensive process.